1. Field of the Invention
The present invention relates to a process of producing a silicon wafer. More particularly, the invention relates to a process of producing a silicon wafer that can have within-wafer uniformity of BMD (Bulk Micro Defect) density even if a ring-shaped OSF (Oxidation Induced Stacking Fault) region has been formed in the course of growing a single crystal by the Czochralski method.
2. Description of the Related Art
To produce a silicon wafer (hereinafter simply referred to as “wafer”) for use as a substrate in which semiconductor elements are to be formed, the Czochralski method (hereinafter simply referred to as “CZ method”) is generally employed. In the CZ method, a seed crystal is immersed in the melt heated in a crucible and pulled up while it and the crucible are being rotated. With the CZ method, however, it is extremely difficult to make the crystal grow to have uniform characteristic with respect to oxygen concentration.
To attain IG (Intrinsic Gettering) effect that accomplishes the gettering of impurities such as heavy metals present in the crystal surface of the wafer, a specific technique is generally utilized. In this technique, a single crystal grown by the CZ method is sliced into wafers. The wafers are subjected to planarization and then to a high-temperature heat treatment in a reducing atmosphere or inert-gas atmosphere. A denuded zone is thereby formed in the surface of the wafer, to a thickness of about 20 μm from the wafer surface. The BMD density of the wafer is thereby enhanced, too.
In recent years, it has become possible to form semiconductor elements smaller and, thus, in higher integration density. It is therefore increasingly demanded that wafers should have within-wafer uniformity of BMD density.
As known in the art, in the process of growing a single crystal grow by the CZ method, the ratio (V/G) of the speed V of crystal growth to the intra-crystal temperature gradient G at a temperature immediately below the melting point results in a fault-seed region (i.e., void-fault region, ring-shaped OSF (Oxidation-Induced Stacking Fault) region, non-fault region, or dislocation cluster region). Of these fault-seed regions, a ring-shaped OSF region may be formed. In this case, as known in the art, a BMD region having a remarkably low density of precipitated oxygen is formed in the ring-shaped OSF region (i.e., high-BMD density region) and in the vicinity thereof. Consequently, two regions, i.e., a high-BMD-density region and a low-BMD-density region, are formed in the surface of any wafer that has a ring-shaped OSF region formed during the growth of the single crystal. Such a wafer will inevitably have non-uniform BMD density in the surface even if it is heat-treated at high temperature. The low-BMD-density region of the wafer reduces the lifetime and oxide-film withstand voltage and, ultimately, lower the yield of semiconductor devices made by processing the wafer.
A method which improves the within-wafer uniformity of BMD density is available. The method is to control the ratio V/G, thereby to position the ring-shaped OSF region in the outer peripheral part of the wafer in the center part thereof. To achieve this control of V/G, the hot zone must be appropriately shaped or the seed crystal must be pulled up at a precisely correct speed. The control of V/G is difficult to perform successfully and requires high cost.
A technique has been proposed, which may increase the BMD density within wafers. In this technique, a single crystal made by the CZ method and having nitrogen concentration of 0.01×1015 to 5.0×1015 atoms/cm3 and oxygen concentration of 0.7×1018 to 1.4×1018 atoms/cm3 is heat-treated for 10 minutes or more at 1000 to 1400° C. (See, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2001-284362.)
Another technique has been proposed, in which a single crystal having nitrogen concentration of 1×1010 to 5.0×1015 atoms/cm3 and oxygen concentration of at most 1.2×1018 atoms/cm3 is heat-treated at a temperature equal to or lower than the melting point of silicon. (See, for example, Jpn. Pat. Appln. Laid-Open Publication No. 11-322491.)
These patent publications disclose processes of producing single-crystal silicon wafers that have high BMD density and exhibit sufficient IG effect. However, they neither disclose nor suggest any means for enhancing the within-wafer uniformity of BMD density of a wafer that has a ring-shaped OSF region formed in the surface.